Typically, a humidity sensor measures the humidity level by measuring the change in the resistance of an element or the change in the electrostatic capacity of that element as it absorbs or releases moisture. Generally, humidity sensors can be classified as belonging to either one of two classes--a resistance type (or resistance-variation type) humidity sensor and a capacitance type (or capacitance-variation type) humidity sensor. A resistance type humidity sensor detects relative humidity by measuring the change in the resistance of an element corresponding to the ambient humidity. In comparison, a capacitance type humidity sensor detects humidity by measuring the change in the electrostatic capacity of an element corresponding to the ambient humidity. The capacitance type humidity sensors typically do not exhibit a satisfactory linear relationship between the capacitance and humidity, and an external circuit is required to overcome this disadvantage. This increases the manufacturing cost of capacitance-change type humidity sensors. Thus, the resistance type humidity sensors, which generally exhibit a linear relationship between the resistance and humidity, appear to have been the preferred choice.
Most of the resistance type humidity sensors include an electrolytic, polymeric, or metallic oxide sensor element. An electrolytic sensor element, which has become the most predominant type of humidity sensors, is made by forming a layer of moisture-lyzable (i.e., can be hydrolyzed by moisture) electrolyte on an insulating moisture-absorbing substrate. U.S. Pat. No. 4,635,027 discloses a resistance-variation type moisture sensor, which comprises a moisture sensitive film made of a moisture sensitive material coated on an insulating substrate, such as alumina or glass. The moisture sensitive material consists essentially of sodium styrenesulfonate, methylene-bis-acrylamide, polyvinyl alcohol, and polyethylene glycol. The polyethylene glycol is contained in the moisture sensitive film in an amount ranging from 3 to 7 parts by weight per 100 parts by weight of sodium styrenesulfonate.
U.S. Pat. No. 5,001,453 discloses a humidity sensor which includes an insulating substrate, a pair of electrodes formed on the insulating substrate, and a porous silica film with carbon particles dispersed therein formed over the insulating substrate and electrodes. A silica film can be formed over the porous silica film, either directly on the porous silica film containing the carbon particles, or directly on the insulating substrate with the electrodes formed thereon to increase adhesion between the porous silica film and the substrate.
The above mentioned humidity sensors involve moisture-absorbing materials, which, after long term usage in a high humidity environment, such as in certain high humidity applications or when used in high humidity areas such as South and Southwest Texas and Florida, will experience property degradation. Also, these materials are thermally unstable and will be subject to instant destruction when exposed to high temperature environments. Additionally, these materials also lack structural integrity and desired ruggedness.
In summary, most of the today's humidity sensors contain a humidity sensing element made from one of the following materials: (a) a moisture-lyzable film containing a electrically conductive material (such as carbon particles); (b) a moisture-absorbing film containing an electrolyte material (such as sodium chloride); and (c) a polymeric electrolyte film. Type (a) materials are known to exhibit the disadvantages of having relatively narrow ranges of measurable humidity changes, because their resistances vary significantly with humidity especially at high humidity environments, and their relatively low sensitivity at low humidity environments. Type (b) materials also have relatively narrow ranges of measurable humidity changes. In addition, type (b) materials are not suitable for long term use at high humidity environments, because the electrolytes contained therein can be diluted and lost due to excessive moisture absorption. Type (c) materials can experience permanent damage at elevated temperatures. However, this problem is also shared by types (a) and (b) materials. All of types (a), (b), and (c) materials will experience gradual property degradation at high humidity environments. Another problem experienced by these humidity sensors is that non-linearity or different slopes of linearity may be experienced at different ranges of relative humidities. This problem is illustrated in the sensor material disclosed in the '027 patent.
Because of their superior linearity between the measured resistance and relative humidity, metal oxides and ceramics may be advantageously used as the sensing material for making humidity sensors. Metal oxides also provide the advantages of having desired ruggedness, durability, improved temperature and chemical resistances, and their ability for long term use at high-humidity environments. Those metal oxides that may be used for this application include the porous Ba.sub.0.5 Sr.sub.0.5 TiO.sub.3, Li.sub.5 AlO,.sub.4 Li G.sub.5 aO, .sub.4 TiO/.sub.2 SnO, .sub.2 .beta.-Fe O,.sub.2 etc.sub.3. When metal oxides are used as the humidity sensing material, their quality is determined not only by the material so chosen, but also by the manufacturing process, including the sintering process. Metal oxides provide a humidity-sensitive electrical conductivity based on the principle that hydrogen ions (H.sup.+ or H.sub.3 O.sup.+) are conducted in the porous structure of the solid state conductor, and the relative humidity value can be measured based on the change in resistance between two spaced apart electrodes as a result of the change in the humidity of the surrounding environment.
Since metal oxides offer many potential advantages as the sensing material for humidity sensors, it is desirable to explore potential candidates that can provide improved performance as well as reduced cost and reduced impact on the environment (both during manufacture and disposal after spent).
One of the possible candidates for use as humidity sensing materials is tungsten oxide, which has been known to exist in several structures including: cubic, hexagonal, orthorhombic, monoclinic, and triclinic crystalline structures, and non-crystalline structures. Among these various types of tungsten oxide, the cubic-structured tungsten trioxide, which has a crystalline structure similar to that of pyrochlore (and hence is called in the present invention "pyrochlore-type tungsten trioxide"), is the one with the most capacious structure (i.e., with the largest amounts of free space and three-dimensional interconnection of tunnels). The pyrochlore-type tungsten trioxide has a general formula of (M.sub.2 O).sub.x WO.sub.3.zH.sub.2 O, where M is a cation, and z is the amount of crystalline water.
The pyrochlore-type tungsten trioxide, whose structure is shown in FIG. 1, has a large number of cavities and tunnels. These cavities and tunnels would facilitate the mass transport of cations therewithin, and allow the pyrochlore-type tungsten trioxide to provide useful applications in ionic exchange, as an ionic conductive material, and in reduction-oxidation-type intercalation reactions. However, some of the undesirable characteristics of the pyrochlore-type tungsten trioxide caused it to be excluded from consideration as a candidate for making solid state humidity sensors. The main reason is that pyrochlore-type tungsten trioxide will be transformed into a different crystalline phase when it is heated to temperatures of about 350.degree. C. or higher. Most of the coating processes for making metal oxide films involve heating the coating material to very high temperatures, typically in excess of 350.degree. C., so as to cause it to vaporize and be deposited on the surface of the substrate. During this heating process, the pyrochlore-type crystalline structure of tungsten trioxide would have been destroyed, thus rendering it impossible for use as a sensing element in a humidity sensor.